2.0 Analysis 2.1 Introduction The weather and terrain were not considered factors in this occurrence. The analysis will address the failure of the aft short tail rotor drive shaft, and the effect of the loss of tail rotor thrust in a hover. 2.2 The Aft Short Tail Rotor Drive Shaft Failure Available maintenance records indicated that the aft short tail rotor drive shaft had accumulated approximately 19,000 hours of time-in-service. Most of that time was accrued in a highly corrosive, salt water operating environment. The drive shaft had been improperly reworked at some time in the past, and the anodizing and lacquer surface protection on the interior wall had been scratched and penetrated. It is probable that the scratches allowed the initiation of the corrosion, and that the prolonged exposure to an extremely corrosive operating environment hastened the spread of the corrosion within the bore of the drive shaft. The corrosion went undetected, as it was not visible on the outer wall and there is no scheduled requirement for an internal visual or NDT examination. The aft short tail rotor drive shaft eventually failed under normal loading, after being weakened by the presence of intergranular cracking which extended from the corrosive attack of the interior wall. The failure may be an isolated occurrence as no record of a similar aft short tail rotor drive shaft discrepancy or failure was found. 2.3 The Loss of Tail Rotor Thrust The altitude and low airspeed at which the helicopter was operated when the loss of tail rotor thrust occurred were within the avoid operation area of the height/velocity specified by the manufacturer, and precluded a successful autorotation. The emergency was compounded by the perilous location of the ground assistant, who was standing under the helicopter as it began to rotate uncontrollably. The pilot successfully manoeuvred the gyrating helicopter away from the ground assistant and the generator shed before impact; however, it is probable that this procedure aggravated and prolonged the uncontrolled flight, and may have contributed to the roll-over which occurred at impact. The dragging of the unreleased sling load may also have aggravated the loss of control, and contributed to the roll-over event. It is probable that the pilot's use of a protective helmet prevented serious head injury. 3.0 Conclusions 3.1 Findings The aft short tail rotor drive shaft failed in an area weakened by intergranular cracking which had progressed from corrosion on the interior wall of the drive shaft. Analysis of the corrosion product deposits disclosed the presence of elements such as sodium, sulphur, chlorine, potassium, and calcium, which are the major constituents of sea water. Historical records indicated that the drive shaft had operated for many years in a warm, humid, and highly corrosive salt water environment. The corrosion which existed in the bore of the drive shaft was not visible with the drive shaft installed. The failure of the aft short tail rotor drive shaft resulted in a loss of tail rotor thrust. The failure occurred while the helicopter was hovering at an altitude which precluded a successful autorotation. The aft short tail rotor drive shaft had been improperly reworked at some time in the past. The aft short tail rotor drive shaft is not a serialized item, and there is no requirement to maintain an applicable component history record. The pilot successfully manoeuvred the gyrating helicopter away from the ground assistant and the generator shed before impact. The pilot was not wearing the available shoulder harness. It is probable that the pilot's use of a protective helmet effectively prevented serious head injury. The pilot did not release the sling load prior to impact. It was not determined why the oil cooler impeller vanes detached from the supporting end plates. 3.2 Causes The aft short tail rotor drive shaft failed in an area weakened by intergranular cracking which initiated from corrosive attack on the interior wall of the shaft. The drive shaft failure resulted in a loss of tail rotor thrust. The failure occurred when the helicopter was hovering at an altitude which precluded a successful autorotation. The Board has no aviation safety recommendations to issue at this time.4.0 Safety Action The Board has no aviation safety recommendations to issue at this time.